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The soldier whose Fort Hood shooting rampage Wednesday killed three people may have argued with another soldier just beforehand, and investigators believe his unstable mental health may have been an underlying cause, military officials said Thursday. (Published Thursday, Apr 3, 2014)

Updated at 1:18 PM EST on Friday, Apr 4, 2014

The soldier whose Fort Hood shooting rampage killed three people Wednesday may have argued with another soldier just beforehand, and investigators believe his unstable mental health contributed to his attack, military officials said Thursday.

The commanding general at Fort Hood, Lt. Gen. Mark Milley, positively identified the gunman in Wednesday's mass shooting as 34-year-old Spc. Ivan Antonio Lopez, a veteran of the Iraq War, in a news conference Thursday afternoon.

Fort Hood Official Gives Info on Suspected Shooter

Milley said there is a "strong possibility" that Lopez had a "verbal altercation" with one or more soldiers immediately before the shooting.

There is no evidence Lopez was targeting any specific victims, Milley said, and investigators have not determined whether the attack were premeditated. There are no indications Lopez was linked to any terrorist group, Miller said, adding that authorities will let their investigation run its course.

Officials said Lopez's unstable mental health is believed to be an underlying cause of the attack.

The Army Criminal Investigation Command is leading the investigation into the shooting. Milley said they are currently synchronizing all efforts throughout Fort Hood and the surrounding area, and that the investigation is ongoing and active.

He was an automatic rifleman from 2010 until 2013, and in December, he switched his specialty to truck driver, the job he held during his time in Iraq. He arrived at Fort Hood in February.

Among Lopez's awards and decorations during his military career were two Army Commendation Medals, four Army Achievement Medals, three Army Good Conduct Medals, two National Defense Service Medals, the Iraqi Campaign Medal with campaign star, the Global War on Terrorism Expeditionary Medal, the Global War on Terrorism Service Medal and more.

Lopez joined Puerto Rico's National Guard in 1999. He went on a peace and security mission to Egypt's Sinai Peninsula in the mid-2000s, and left the National Guard in 2010 to join the U.S. Army, said Lt. Col. Ruth Diaz, spokeswoman for the Puerto Rico National Guard.

Lopez was originally from the town of Guayanilla, Puerto Rico, according to military officials, and NBC News reported that he was a percussionist. His mother died in November, one month after his grandfather died.

Guayanilla Mayor Edgardo Arlequin Velez told the Puerto Rican radio station Radio Isla 1320-AM that he used to teach Lopez as a band director. His former student was "a peaceful young man and especially talented," he said.

Gunman's Mental Health Under Scrutiny

Lopez was examined by a psychiatrist last month and was found to show no violent or suicidal tendencies, Army Secretary John McHugh said. Lopez had been prescribed Ambien to deal with a sleeping problem.

Lopez had also sought help for depression, anxiety and other problems, military officials said. He was being evaluated for post-traumatic stress disorder but had not yet been officially diagnosed.

He was taking medication, and there were reports that he had complained after returning from Iraq about suffering a traumatic brain injury, Milley said.

NBC News reported Lopez's deployment in 2011 was at a time when the U.S. military was in the process of withdrawal from Iraq. Military officials told NBC News that does not mean Lopez did not suffer psychological problems, but only that they did not appear to be combat-related.

Update on Wounded; Memorial to be Held Next Week

Four people remain hospitalized at Baylor Scott & White Hospital in Temple, after five patients were discharged Thursday. Three of those still hospitalized there were upgraded Thursday from critical to serious condition, and then upgraded again to fair condition on Friday. The fourth patient was in good condition and would likely be discharged Friday.

Another three victims were being treated at Darnall Army Community Hospital at Fort Hood.

"I can't tell you how honorable it is to have the opportunity to care for our military," Dr. Stephen Sibbitt, chief medical officer for Baylor Scott & White Memorial, told NBC 5 DFW on Thursday.

"It's a very unfortunate circumstance, but it's incredibly humbling to see how they conduct themselves even in the face of such injuries. They conduct themselves with the highest level of professionalism and stature. It's something we all need to be very proud of, is our military."

Milley added that there will be a memorial service held at the post next week to remember those whose lives were lost in the shooting. The time and date for the memorial not yet been set.

Investigators Speak With Gunman's Wife

Authorities reached out to Lopez's wife Wednesday night and were expected to search his home and any computers he owned.

Lopez's neighbors in Killeen told NBC 5 that he moved into the apartment complex about three weeks ago with his wife and young daughter.

"They seemed real sweet," said neighbor Xanderia Morris.

Morris said when Lopez's name was announced on TV news reports that his wife came out of her apartment hysterical. Morris said she comforted her until authorities arrived a short time later and escorted her away. Their daughter apparently left with relatives, Morris said.

A woman who answered the door to the apartment Thursday morning nodded when asked if she was the wife of the suspected gunman.